Git - could not checkout remote branch

When you specify no branches, the default settings are used. The default means to fetch and update all branches existing in the remote repository.

See documentation for details:

git pull [options] [<repository> [<refspec>…​]]

<refspec> specifies which refs to fetch and which local refs to update. When no <refspec>s appear on the command line, the refs to fetch are read from remote.<repository>.fetch variables instead (see git-fetch[1]).

You often interact with the same remote repository by regularly and repeatedly fetching from it. In order to keep track of the progress of such a remote repository, git fetch allows you to configure remote.<repository>.fetch configuration variables.

Typically such a variable may look like this:

[remote "origin"]
fetch = +refs/heads/*:refs/remotes/origin/*

The example above will fetch all branches that exist in the origin (i.e. any ref that matches the left-hand side of the value, refs/heads/*) and update the corresponding remote-tracking branches in the refs/remotes/origin/* hierarchy.

The behavior is default because it allows you to synchronize the whole repositories at once. If you don’t want to update all local branches at once, use git fetch to synchronize the repositories and git merge origin/<branch> to update each single local branch.